A. Field of the Invention
The present invention relates to a method of stimulating a subterranean formation penetrated by a wellbore, particularly where a mineral acid is injected into the formation to dissolve a portion of the formation, especially as a part of a fracture acidizing treatment.
B. Description of the Prior Art
The problem of decreasing the permeability of certain portions of a subterranean formation so that certain treatments thereof with aqueous fluids can be made more efficient is well known in the art. For example, when acidizing a heterogenous permeable formation, it is important to contact the less permeable zones with the acid. Many additives, generally known as diverting agents, have been developed for this use. Also, when fracturing formations, more efficient treatments can be achieved if the fracturing fluid is not lost to the formation but instead can be further utilized to increase the length and width of the fractures.
Reasonably successful treatments have been performed, particularly at fracturing rates and pressures, wherein a gelled aqueous fluid is injected ahead of an acidizing fluid. However, such treatments have not been as successful as might have been desired, apparently because of high fluid loss. Laboratory studies suggest the acid forms wormholes through the more permeable parts of the formation. Thus, it is believed much of the acid is lost through the fracture walls rather than by penetration to the extremities of the fracture.
The fluid loss problem has been mitigated by utilizing fine particulate materials in the gelled pad fluid. However, the fine particulate has given rise to an unforseen problem. When some wells have been placed back on production, the fine particulate has tended to become entrained in the produced fluids, thereby causing undue wear on equipment, and the like. Thus, while larger size particles used as propping agents tend to stay in the fractures, the smaller size particles used as fluid loss agents apparently have greater mobility in the fracture.
The present invention provides fluid loss control without the need to include particulate material in either the pad or the acid.
As heretofore indicated, it has been known to employ a gelled pad ahead of acid. It had also been known to employ a salt containing fluid ahead of acid. See, for example, Grebe, U.S. Pat. No. 1,916,122; and Broaddus et al., U.S. Pat. No. 3,954,142, and related SPE Paper 5629. Broaddus et al. suggest at column 5, lines 18-38 that it may be desirable to employ viscous fluids of differing density for acid placement control by underriding or overriding of the various fluids. Graded rock salt has been employed as a particulate solid for fluid loss control, see SPE Paper 2751 and Howard et al., Hydraulic Fracturing, page 96 (1970), but not as a solute. There is no known suggestion to employ a dissolved salt in the pad fluid when fluid loss control is desired nor any recognition that salt dissolved in the pad improves fluid loss control.